Include Github repo link in `--help` output 🙂

I had this tool installed locally and wanted to file a feature request (which I'd be happy to contribute!) -- but it took me a while to figure out where I downloaded the tool from! 😅 

It'd be nice if you referred to https://github.com/yukithm/json2csv in the `-h`/`--help` (or at least `--version`) output of the `json2csv` binary.